The invention has been developed in connection with mixing oil sand in hot water. While not limited to that application, it will now be described in connection therewith.
Bitumen, a heavy oil, is currently being extracted on a commercial basis from oil sand. Presently, two very large scale commercial operations are producing synthetic crude oil from oil sand in the Fort McMurray district of Northern Alberta.
At each of these operations, the oil sand is stripmined and conveyed on conveyor belts, often several kilometers in length, to an extraction plant. At the extraction plant, the bitumen is separated from the solids and recovered. This is accomplished using a process known as the `hot water process`.
The hot water process involves mixing the oil sand with hot water (95.degree. C.) and a small amount of caustic in a rotating horizontal drum (or `tumbler`). Steam is added to the mixture as it moves through the tumbler, to ensure that its exit temperature is about 80.degree. C. In the tumbler, the bitumen is separated from the solids, lumps of the cohesive oil sand are ablated and disintegrated and minute flecks of freed oil coalesce to form larger globules. The term "conditioning" is used to denote the sum of the mechanisms occurring in the tumbler. On leaving the tumbler, the slurry is diluted with additional hot water and retained under quiescent conditions for a prolonged period in a thickener-like vessel referred to as a primary separation vessel ("PSV"). In the PSV, the bitumen globules attach to and film around bubbles of air entrained in the slurry and rise to form froth on the surface of the vessel contents. This froth is recovered. A dragstream is withdrawn from the central part of the PSV and this dragstream is processed in a bank of sub-aerated flotation cells to produce a secondary yield of bitumen froth. The froth streams are combined and further processed to remove entrained water and solids and yield essentially pure bitumen.
Now, the belt conveyors are characterized by a number of problems. They are expensive to install, operate and maintain And their use requires that the solids, which have no value, must be conveyed to the extraction plant and then returned by truck to the mine pits for disposal. In addition, the tumblers cannot be increased in size to permit of improvement of the system. They are presently so large that it would be technically difficult to manufacture them in a larger size and convey them to the plant site. As a result, it is difficult to reduce the heat requirements of the process by lowering the slurry temperature, because such a step would require increasing the tumbler retention time, which would necessitate larger tumblers.
In a co-pending application, applicants teach use of a pipeline to convey an aqueous slurry of the oil sands from the mine site to the extraction plant. The pipelined slurry may be fed directly to the PSV, thereby eliminating the need for the tumbler. The invention in the co-pending application is based on the discovery that the slurry will undergo adequate conditioning in the pipeline over a distance that is significantly shorter than the length of pipeline needed to get it to the extraction plant. In addition, the slurry will not be over-conditioned if it continues to move through the pipeline after conditioning is complete. (Conditioning is considered to be complete if good bitumen recovery in the form of good quality froth can be achieved in the downstream PSV). This pipeline scheme has the further advantage that most of the coarse solids may be removed in a settler positioned part way along the length of the pipeline.
So pipelining of the oil sand in slurry form between the mine and the PSV is now considered by applicants to be a viable procedure.
The present invention is directed toward providing a mixer circuit which satisfactorily blends the oil sand with hot water to yield a consistent, dense (e.g. about 60%-65% by weight solids) slurry, preferably having a relatively low temperature (e.g. 50.degree. C.), that is amenable to pipeline conveyance.
In this connection, it needs to be appreciated that oil sand is tacky, cohesive, erosive material incorporating a significant content of "oversize". Oversize is a term applied to the rocks, oil sand lumps, and clay lumps that occur in oil sand (often up to a size of 20 inches).
If one were to feed a stream of oil sand into a tank containing hot water and proceed to withdraw a mixture from the base of the tank with a pump, the oil sand would simply pile up in the tank, fill it, and plug the pump. So a mixer circuit for this purpose must be capable of suspending the oil sand in the water with which it is mixed.
It has been mentioned that it is desirable to produce a dense slurry. This need arises from the fact that one wants to minimize the amount of hot water supplied at the mine site for this purpose. Heating water is expensive and there are many reasons why these plants need to conserve water to the maximum.
And of course the mixer circuit has to be capable of coping with the oversize material. Equipment having moving parts, such as a tank equipped with paddle mixers, would be inappropriate for use with the erosive sand associated with oversize chunks.